Detail Description of the Invention
This invention relates to a method of producing an amorphous Si film by using plasma CVD (Chemical Vapor Deposition) techniques.
The amorphous Si films have been used as solar cells or semiconductor layers of photosensitive materials for electrophotography, and have been produced by plasma CVD techniques in general, that is, by decomposing monosilane (SiH.sub.4) or tetrafluorosilane (SiF.sub.4) by means of low pressure glow discharge. As techniques for generating the glow discharge, there have been proposed a DC type for generating the discharge by arranging electrodes within a vacuum reaction chamber, or a condenser coupling type due to high frequency electric field and an inductive coupling type for generating the discharge by arranging a high frequency coil outside the reaction chamber. However, since it is difficult in the inductive coupling type to establish the uniform electric field, the techniques for arranging electrodes within the reaction chamber have been mostly adopted in industrial plasma CVD equipment. In this case, one electrode acts as an anode and the other acts as a cathode by self-bias in the condenser coupling type which also includes a high frequency electric field of AC voltage with superimposed DC voltage as well as the high frequency electric field. FIG. 1 shows conceptually a reaction chamber 3 having parallel-plate electrodes 1,2. A reaction gas 5 is introduced into the chamber 3 through a supply pipe 6, while vacuum exhausting it through an exhaust pipe 4, and the glow discharge is generated by applying DC voltage between electrodes 1 and 2 through a power source 7, or by applying thereto DC high frequency voltage so that the electrode 1 grounded between electrodes 1 and 2 becomes positive and the electrode 2 becomes negative. In this case, the production of the amorphous Si film is not carried out simply by the electric field, but also takes part adsorption and decomposition reactions of the electrode surface. Therefore, positive Si ions generated by the decomposition of the reaction gas due to the plasma are not deposited on the electrode 2 of the cathode, but also deposited on the electrode 1 of the anode. A large quantity of Si ions is properly deposited on the cathode as compared with the anode but the deposition amount of Si ions on the anode is not disregarded. When a high density amorphous Si film is deposited on a substrate which is placed on the cathode 2 and is heated to about 200.degree. C. by a heater (not shown), the Si film deposited on the anode 1 is entirely useless to decrease the yield.
FIG. 2 shows a manufacturing apparatus for producing an amorphous Si film, used for a photosensitive material for electrophotography, on a cylindrical substrate. A rotatable column-shaped support 8 is provided on a base, opposing to a semi-circular anode 11. If a voltage is applied through a power source 7 to a heated cylindrical substrate 12 formed on the support 8 by using the substrate 12 as a cathode, Si film deposited on an anode 11 is useless to reduce the yield.